Charles G. Taylor, the former president of Liberia and once a powerful warlord, was convicted by an international tribunal on Thursday of 11 counts of planning, aiding and abetting war crimes committed in Sierra Leone during its civil war in the 1990s. He is the first head of state to be convicted by an international court since the Nuremberg trials after World War II.

The ruling, announced by Presiding Judge Richard Lussick of Samoa, said Mr. Taylor was guilty of involvement in crimes against humanity and war crimes, including murder, rape, slavery and the use of child soldiers. But the court said the prosecution failed to prove that Mr. Taylor had direct command responsibility for the atrocities in the indictment.